pioctl: limit fruitless token searches

pioctl: limit fruitless token searches

getNthCell searches the afs_users table for the nth token set belonging to a
given user.  However, it is impossible for a user to have more than one
token set per cell.  If the caller specifies a number greater than the
total number of cells this cache manager knows about, we know the search
will be fruitless.

Instead, return early in this case, avoiding both the lock and the
search.

Change-Id: I509408d9aaa8f511813c4d82c121e199121bb8f3
Reviewed-on: https://gerrit.openafs.org/13597
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>